The present invention relates to a novel catalyst composition, and method of manufacturing same, for the dehydrocyclodimerization of C.sub.2 to C.sub.5 aliphatic hydrocarbons. Additionally a process for producing aromatics via the dehydrocyclodimerization reaction, which utilizes the subject catalyst composition, is disclosed.
Dehydrocyclodimerization is a reaction where reactants comprising paraffins and olefins, containing from 2 to 5 carbon atoms per molecule, are reacted over a catalyst to produce primarily aromatics with H.sub.2 and light ends as by-products. This process is quite different from the more conventional reforming or dehydrocyclization process where C.sub.6 and higher carbon number reactants, primarily paraffins and naphthenes, are converted to aromatics. These aromatics contain the same or less number of carbon atoms per molecule as the reactants from which they were formed, indicating the absence of reactant dimerization reactions. In contrast, the dehydrocyclodimerization reaction results in an aromatic product that always contains more carbon atoms per molecule than the C.sub.2 to C.sub.5 reactants, thus indicating that the dimerization reaction is a primary step in the dehydrocyclodimerization process. Typically, the dehydrocyclodimerization reaction is carried out at temperatures in excess of 500.degree. F. using dual functional catalysts containing acidic and dehydrogenation components. These catalysts include acidic amorphous aluminas which contain metal promoters. Recently crystalline aluminosilicates have been successfully employed as catalyst components for the dehydrocylodimerization reaction. Crystalline aluminosilicates generally referred to as zeolites, may be represented by the empirical formula EQU M.sub.2/n.Al.sub.2 O.sub.3.xSiO.sub.2.yH.sub.2 O
in which n is the valence of M which is generally an element of Group I or II, in particular, sodium, potassium, magnesium, calcium, strontium, or barium and x is generally equal to or greater than 2. Zeolites have skeletal structures which are made up of three dimensional networks of SiO.sub.4 and AlO.sub.4 tetrahedra, corner linked to each other by shared oxygen atoms. The greater the proportion of the SiO.sub.4 species to the AlO.sub.4 species, the better suited the zeolite is for use as a component in dehydrocyclodimerization catalysts. Such zeolites include mordenite and the ZSM variety. In addition to the zeolite component, certain metal promoters and inorganic oxide matrices have been included in dehydrocyclodimerization catalyst formulations. Examples of inorganic oxides include silica, alumina, and mixtures thereof. Metal promoters such as Group VIII or Group III metals of the Periodic Table, have been used to provide the dehydrogenation functionality. The acidic function can be supplied by the inorganic oxide matrix, the zeolite or both.
Molecular hydrogen is produced in a dehydrocyclodimerization reaction as well as aromatic hydrocarbons. For example, reacting a C.sub.4 paraffin will yield 5 moles of hydrogen for every one mole of aromatic produced. Because the equilibrium concentration of aromatics is inversely proportional to the fifth power of the hydrogen concentration, it is desired to carry out the reaction in the absence of added hydrogen. Adherence to this practice, however, promotes rapid catalyst deactivation and, as a result, short catalyst life expectancy. The rapid deactivation is believed to be caused by excessive carbon formation (coking) on the catalyst surface. This coking tendency makes it necessary to frequently perform costly and time-consuming catalyst regeneration. Reducing the coking and thereby increasing catalyst life is the particular object to which this application is directed.
It has now been found that, if a catalyst is formulated with the components, and in the manner set forth hereinafter, an improved process for the dehydrocyclodimerization of C.sub.2 -C.sub.5 aliphatic hydrocarbons will result. The improvements being longer catalyst life expectancy, greater aromatic hyrocarbon yield, and lower catalyst coke levels.